I had Jim Webb check the cowl plenum duct screws in his red '67 rally sport Z-28. His screws look prettymuch the same as Paul Lasiter's although Paul's have four tiny bumps in a line across the head of the screwand it looks as though Jim's might only have two bumps. There are no other markings on the screws.

Here's a closer look at the groove that the top lip of the air cleaner base rides in. There is a very similarlooking lid that you will see at swap meets that is close to the same size (around 17" dia) that has rubberin the groove. Don't be fooled into buying it. It won't fit on an original GM cowl plenum air cleaner base.

Yes, you're right. 1963 was the first use by Chevrolet for this style of air induction. The W-series (409 style) 427 Z-11engine used a modestly different looking cowl plenum air cleaner than the MKII "Mystery Motor" Impalas used at Daytona.

There's no doubt it's a benefit but I haven't seen published figures that show what the horsepower gain was.There would be more benefit on a course with longer straights where a car sustained higher speeds for longertime (Daytona and Sebring) than there would be for shorter tracks with slower speeds.